Only some FMS/RNAV units are approved for IFR navigation,
and it is important to make this determination before flying
with any particular unit. Sometimes, this limitation is based
on the installation (i.e., method of installation, qualifications
of installer), aircraft approval, availability of approved
maintenance, and geographic location. No hand-held GPS unit is approved for IFR navigation, and many panel-mounted
units are restricted to VFR use only.

Even when an FMS is approved for IFR, the installation of
the system in that specific aircraft must also be approved.
Even if you have an IFR-approved FMS unit, you may not
use it for IFR navigation unless the installation is approved
as well. This approval process usually requires a test flight
to ensure that there are no interfering inputs, signals or
static emanating from the aircraft in flight. RNAV units that
do not meet all of these requirements may still be used as
situation enhancing navigation resources when operating
under instrument flight rules.

The first place to check when determining IFR certification
for an FMS is the Pilot’s Operating Handbook (POH) or
Aircraft Flight Manual (AFM). For every aircraft with an
IFR approved FMS/RNAV unit, the AFM explicitly states
that the unit has been approved for IFR navigation and what
IFR operations are specifically authorized for that installation.

Navigation Database Currency

The navigation database contained in the FMS must be
current if the system is to be used for IFR navigation and
approaches. Some units allow en route IFR operations if the
navigation waypoints are manually verified by the pilot and
accepted. The effective dates for the navigation database
are shown on a start-up screen that is displayed as the FMS cycles through its startup self-test. Check these dates to ensure
that the navigation database is current. Figure 3-7 shows the
start-up screen and effective dates for one popular FMS.

Alternative Means of Navigation

To use some GPS-based RNAV units (those certified under
Technical Standard Order (TSO) 129) for IFR flight, an
aircraft must also be equipped with an approved and alternate
means of IFR navigation (e.g., VOR receiver) appropriate
to the flight. Ensure that this equipment is onboard and
operational, and that all required checks have been performed
(e.g., 30-day VOR check).

The avionics operations manual/handbook should state the
certification status of the installed system. The supplements
to the AFM should state the status of the installed equipment,
including the installed avionics. Most systems require that the
advanced avionics manuals be on board as a limitation of use.

NOTAMs Relevant to GPS

There are numerous notices to airmen (NOTAMs) that
apply specifically to users of navigation aids. For example,
when anomalies are observed in the behavior of the global
positioning system, or when tests are performed, a GPS UNRELIABLE NOTAM is issued. Similarly, published
instrument procedures that rely on RNAV equipment
sometimes become “Not Available” when safety concerns
arise, such as ground-based interference. It is important to
check all NOTAMs prior to IFR flights and, especially,
GPS and WAASNOTAMs before flying. Remember, when
talking to a flight service station (FSS)/automated flight
service station (AFSS) briefer, you must specifically request
GPS/WAAS NOTAMs.

GPS Signal Availability

GPS-based RNAV equipment that uses the DOD GPS relies
on adequate signal reception throughout the course of a
flight. Signal reception becomes especially critical during
instrument approaches when signal reception criteria become
more stringent. Signal reception is generally predictable, and
you can request information on likely signal reception for
the destination airport in the preflight briefing from Flight
Service. Many GPS RNAV units include a feature called
receiver autonomous integrity monitoring (RAIM) that
allows you to view predictions about future signal reception
at specific locations. WAAS-enabled receivers do not have
this restriction or limitation due to the error corrections
available from the WAAS. WAAS is a form of differential
GPS (DGPS) providing enhanced position accuracy. Each
Wide Area Reference Station (WRS) provides correction
data to a Wide Area Master Station (WMS), which computes
a grid of correction data to be uplinked to a geostationary
satellite (GEO) from a Ground Earth Station (GES). The
geostationary satellite transmits the correction data (and
also navigation data) to the user on the L1 GPS navigation frequency (1575.42 MHz). The user GPS receiver uses the
downlink WAAS data to correct received navigation data.
The goal of WAAS is to obtain at least a 7-meter horizontal
and vertical accuracy.

Local Area Augmentation System (LAAS), when it becomes
available, is another DGPS mode which is designed to
provide 1-meter accuracy for precision approaches. It uses
a local error VHF transmitter near the runway providing a
direct link from the sensor to the aircraft GPS receiver.